In recent years, liquid crystal displays are used not only for monitors for personal computers but also for TVs, and in the liquid crystal displays, techniques to enlarge the size and techniques to make luminance higher are developed, and demands for a thinner size are increasing. Accordingly, it is taken for granted that high durability is required, and further, demands for a thinner size for each component used for the liquid crystal displays are increasing.
Meanwhile, conventionally, as a retardation film used for improving visibility and view angle characteristics in a liquid crystal display, the common techniques dispose a polycarbonate film with properties exhibiting highly the birefringence of the resin itself or a retardation film (retardation plate), in which an optical anisotropy layer including a liquid crystal material is disposed on a cellulose film, between a polarizer and a liquid crystal cell.
However, the polycarbonate film has a poor adhesiveness for a polyvinyl alcohol film which includes biaxiality dyes as an additive, is stretched, and used generally as a polarizer. Problems arise such that when the polycarbonate film is pasted directly to a polarizer, the durability of a polarizer deteriorates, and when an adhesive layer or an inter layer is disposed between a retardation film and a polarizer, the resulting polarizing plate is made thicker.
Moreover, since the retardation film provided with an optical anisotropy layer including a liquid crystal material is made in a two layer structure as a support, the production process becomes complicate, and problems of film thickening arise.
Then, in order to increase the adhesive properties to the polarizer and to provide an excellent isotropy, cellulose ester film that is used conventionally as a polarizing plate protective film is provided with a retardation function. Further, studies are made to thin the polarizing plate and to enhance the durability of the polarizing plate.
Since cellulose triacetate film has originally a high isotropy and weak birefringence exhibiting property, the cellulose triacetate film is not suitable for a retardation film. Then, studies are made for a retardation film so as to add a so-called retardation increasing agent to a cellulose triacetate resin and to employ mixed fatty acid ester of celluloses such as cellulose acetate propionate that has a higher birefringence exhibiting property than a cellulose triacetate resin among cellulose ester resins (for example, refer to Patent Documents 1 and 2).
However, in the retardation film in which the above-mentioned retardation increasing agent is added, in order to make the retardation film thinner, it is necessary to increase an additive amount of the retardation increasing agent per film thickness. As a result, problems called bleed-out may arise such that additive agents may precipitate from film.
Moreover, although the retardation film employing the mixed fatty acid ester film of the cellulose has a high birefringence exhibiting property as compared with cellulose triacetate, the thinning of such a retardation film is required to increase further a stretch ratio. Accordingly, since the degree of difficulty in manufacture is high, there are problems in stable manufacture of the film. Further, since mixed fatty acid ester resin itself of cellulose has the high degree of difficulty in synthesis, problems arise in manufacture of the polarizing plate at low cost.
Then, the present inventors studied to prepare a polarizing plate such that a protective film which employs cellulose ester with the total degree of substitution of 2.1 to 2.6 by acyl groups having a high birefringence exhibiting property, exhibits retardation, and serves additionally as a retardation film is used as a protective film which is disposed at the liquid crystal cell side of a polarizer and is required to have an optical compensation function, and a cellulose ester film with a total degree of substitution of 2.8 to 3.0 and a high isotropic is disposed at the opposite side of the polarizer. This ensures to make the film thinner while maintaining the adhesiveness to the polarizer and to manufacture the polarizing plate with cheaper materials.
However, when the resulting polarizing plate was incorporated so as to produce a liquid crystal display, it was acknowledged that streak-like irregularities slightly occur in images at the time of black indication. Further, when the liquid crystal display has been preserved for a long period of time under a wet heat environment, wavelike irregularities occurred on images and a phenomenon in which visibility deteriorated was observed.
In the case where liquid crystal displays are used as large-sized liquid crystal displays, such as large-sized TV and external monitors, when liquid crystal displays are used with a back light the luminance of which is made high and under an external environment so that the liquid crystal displays are used under severer environment than ever before, the above problems are required to be improved.